1. Field of the Invention
This invention relates in general to a vaporizer apparatus which is used, for example, in chemical vapor deposition of thin films from liquid feed material, and relates in particular to a liquid feed vaporizer apparatus for vaporizing such high dielectric thin film forming materials as barium/strontium titanate.
2. Description of the Related Art
In recent years, significant progress has been made in the field of high density integration of semiconductor electronic circuits in anticipation of achieving giga-bit-capacity DRAMs to overtake the currently available mega-bit-capacity DRAMs. For obtaining the high capacity dielectric thin films necessary to produce such DRAM circuits, the currently used dielectric materials such as silicon oxide and silicon nitride films having dielectric constant less than 10 are to be superseded by metallic thin film materials such as tantrum pentaoxide (Ta.sub.2 O.sub.5) having a dielectric constant in the range of 20, barium titanium oxide (BaTiO.sub.3) or strontium titanium oxide (SrTiO3) having a dielectric constant in the range of 300, or barium strontium titanate which is an admixture of the afore-mentioned compounds.
As a method of growing thin films from such materials, chemical vapor deposition (CVD) is favored. In this case, it is necessary that, a gas-phase feed material be supplied steadily on a substrate material held in a reaction chamber. In order to stabilize the vaporization characteristics, the feed gas is derived by heating for vaporization a liquid source produced by dissolving materials such as Ba(DPM).sub.2 or Sr(DPM).sub.2, which are solid at normal temperature, in some organic solvent such as THF.
It is extremely difficult to stably vaporize feed materials of highly dielectric materials described above because: (1) vaporization and decomposition temperatures are close together; (2) a difference exists in the vaporization temperatures between the film material and the organic solvent; and (3) the vapor pressures are all very low. For example, for a liquid feed made by dissolving Ba(DPM).sub.2 or Sr(DPM).sub.2 in THF, the liquid phase of the solvent ranges in the region marked as "a" in FIG. 6, and the liquid or solid phase of the material ranges in "a+c". Therefore, when raising the temperature of the liquid through the region marked "c" to vaporize the liquid feed which is in "a", there is a danger that only the solvent is evaporated, causing the film material to precipitate out and resulting in plugging of the passage or degradation in the feed quality due to compositional changes.
Therefore it is generally considered that, when vaporizing a liquid feed material, it is necessary to rapidly heat the liquid to bring it quickly to the high temperature region. A known type of vaporizer apparatus utilizes a technology of first preparing atomized mist with the use of an ejector nozzle or ultrasonic vibrator, and then heating the mist in a high temperature region to produce a vapor.
However, in the conventional type of vaporizer apparatus presented above, a porous member loses heat due to the heat of vaporization of the liquid feed dispersed therein, and if a local low temperature zone should be created, it may cause a clogging of the porous member. Therefore, it is necessary that a fairly large amount of excess heat be supplied to the porous member. In the conventional vaporizer apparatus, the heat to the porous member is supplied externally, and the center of the porous member is heated primarily by the heat conducted through the porous member itself. Therefore, the supply of heat to the interior of the porous member tends to be insufficient, and consequently the vaporizing efficiency is lowered due to such problems as clogging in the interior of the porous member.
For the type of vaporizing process in which only the lower temperature limit is important, the exterior temperature of the porous member is heated to a sufficiently high temperature so that the interior region of the porous member can reach the required temperature. However, for certain types of liquid feed material, excessive raising of the external temperature may cause a possible decomposition or degradation of the liquid or evaporated feed material. For such materials, the presently employed method cannot be employed because the required range between the upper and lower temperature limits is too narrow.